The present invention relates to intermediate bulk containers made from fabric material and having improved lifting loops. Such a container includes side walls, at least one lifting loop, which is formed by joining extensions which are integral with the side wall structure, and a base structure.
Such containers are generally manufactured from at least one piece of woven fabric, particularly woven polypropylene or other suitable synthetic material, and are required to carry loads of 500 kg or more with a considerable safety margin. The containers are used for storage and transport of bulk material in granular, power or paste forms. Fabric extensions used for making the lifting loops are integral, continuous extensions of the side walls, but obviously such extensions can be separate pieces of fabric material joined to the side walls by seams.
During handling of the containers, both the lifting loops and the base construction must absorb the resulting stress from static and dynamic loads. The wall structure, however, needs only to absorb the static load during stacking of the container. The lifting loops are the element of the construction which must absorb the highest loads during handling. Strong lifting loops are accordingly the most essential features for a competitive container.
To date such loops have been formed by joining the integral extensions by the fast and inexpensive method of sewing. Tests involving several types of seams proved it impossible to increase the strength of the loop joints substantially unless the fabric strength was increased, thereby requiring heavier and more expensive fabric. Improved sewing techniques have resulted in ever stronger seams, but in spite of that, loops including seams are still weaker than loops formed as continuous integral extensions of the wall structure, i.e. no seams in the loop itself. However strong such presently known loops seem, they constitute a weak part of the container and should accordingly be improved.
Great success has been achieved with the containers according to U.S. Pat. No. 4,136,723 and which have continuous integral lifting loops and seams in the wall and base structure. An alternative arrangement comprises integral extensions of the side wall structure joined by seams to form lifting loops. But these loops are not as strong as the former ones due to the seams.
When two side wall extensions are joined by a seam to form a lifting loop, it is necessary to place the seam somewhat below the extremes of such extensions, for instance about 10 cm. Then the two fabric parts above the seam are folded down at one side of the seam, and the loop is gathered together to form a lifting portion or handle. This way of joining the extensions implies that there will be one layer of fabric on one side of the seam and three layers on the other. Consequently, the seam will be directly exposed to lifting stress during handling of the container, and the lifting loop will tend to slide on the lifting means due to the 1:3 distribution of the layers of fabric around the seam. A further consequence is that when a loop is gathered together to form a lifting portion, such portion or area will not have even thickness.